A. Field of the Invention
This invention relates to self-calibration of ophthalmic instruments and, in particular, objective refractors.
B. Description of the Prior Art
The typical objective refractor has a light source, usually in the infrared band, that provides a beam of light that is reflected by the findus of a patient's eye to a detector. A lens system movable along the instrument axis is used to incrementally shift the image plane from one side of the fundus to the other side. The signal generated by the detector is then evaluated to determine the amount of refractive spherical error in the eye using the position of the lens system when the condition of "best focus" occurred. Rotational movement about the instrument axis is similarly used to determine the amount and axis of refractive cylindrical error. The calibration of these instruments is extremely difficult and time consuming, since the position and orientation of each optical element is critical to obtaining accurate results. In objective refractors that have one or more components on a retro-reflective path, errors introduced on the way to the eye are canceled by the same component(s) as the light reflected by the findus passes through them on the way from the eye. Even after precision alignment, the production instruments still retain some error due to inconsistences in components and their alignment. Such errors are usually "removed" at the factory by compensation using software and/or hardware to report corrected values.
Usually objective refractors have at least one beamsplitter and several paths to be joined to and/or separated from a common path. As those skilled in the art know, beamsplitters transmit part of the light and reflect part of the light striking them and either the transmitted or reflected light is lost.